This invention relates to a microporous polypropylene membrane, in particular to such a membrane having a pore-size rating of 0.1 micron, and to a method for its preparation.
Microporous polypropylene membranes have existed in the art for many years, made by various methods. Of particular interest is the method described in U.S. Pat. No. 4,247,498 for making microporous products, including microporous polypropylene membranes.
Heretofore microporous polypropylene membranes having a pore-size rating of 0.2 microns have existed which have exhibited suitable physical properties. However, for certain applications it is necessary and/or desirable to use polypropylene membranes having an even smaller pore-size rating, such as 0.1 micron. Past efforts to develop a 0.1 micron polypropylene membrane have not been successful. Previous attempts to produce such a membrane have resulted in membranes which either do not meet the pore-size requirement or which do meet the pore-size requirement, but suffer from other physical problems, such as low burst strength or poor flow rate characteristics.
Thus, there has existed a need in the art for a microporous polypropylene membrane having a pore-size rating of 0.1 micron, while being capable of maintaining a good flow rate, as well as possessing other desirable physical properties.